The present invention relates to a wideband microwave device, generating even harmonics of an incident signal.
A frequency doubling circuit is only one particular embodiment of the circuit according to the present invention. Several embodiments of frequency doublers are known among which are those which use a frequency duplexer.
A signal, whose frequency is to be doubled, is injected into the low channel output, corresponding to the low-pass filter, branched to the common channel comprising a non-linear element such as a Schottky diode. The high channel output of the duplexer then supplies the harmonics generated by the non-linear element. The uneven harmonics are rejected by filtering at the output of the high-pass filter of the duplexer. This filtering constitutes one of the main disadvantages of this type of doubler, because it limits the operating frequency band thereof. The insertion losses of the duplexer circuit results in a considerable reduction in the conversion efficiency. Another type of doubler utilizes a mixer. The fundamental signal is supplied to the input of a power divider, whose two outputs having identical frequencies, are applied to the so called "local oscillator" and "signal inputs" of a mixer. The "intermediate frequency" output of the mixer provides a signal having a frequency equal to the sum of the frequencies of the input signals, i.e. double the frequency of the incident fundamental signal.
The circuit formed by the combination of a power divider and a mixer does indeed form a frequency doubler, but has certain disadvantages. It is vital to use a mixer having a very good rejection of the second harmonics between the "local oscillator" input and the output of the mixer in order to be able to isolate the signal, whose frequency corresponds to the sum f.sub.oL +f.sub.s of the input frequencies. Moreover, it is difficult to produce mixers, whose inputs have sufficiently wide bandwidth to ensure an appropriate frequency coverage at the output. The so called asymmetrical/symmetrical has been described in the article by J. KOHLER and B. SCHIECK in "The Radio and Electronic Engineer", Vol. 48, No. 1/2, January/February 1978, pp. 29-32 and entitled "Broadband Microwave Frequency Doublers".
Such a doubler is explained in greater detail in the description relative to FIG. 2. Briefly it comprises a pair of non-linear elements located at the point at which an asymmetrical propagation line forming the input of the circuit and a symmetrical propagation line forming the output meet. The input of the circuit described in this article is a coplanar line, whilst the output is a slotline, the non-linear elements used being Schottky diodes. This circuit, which is simple to structurally configure, has the disadvantage of not being usable in the lowest part of the microwave frequency spectrum, due to the operating limits of the slotline used at the output.